Conventionally, in the field of lubricating oils, improvement of lubricating oils has been studied from the viewpoint of an energy saving property. Especially in recent years, a trend toward the global environmental protection has increased, and a need for an energy saving property improving effect for lubricating oils has been further strengthened.
For example, in the case of lubricating oils used for internal combustion engines such as a vehicle engine (also referred to as “lubricating oils for an internal combustion engine” or “engine oils”), as one means of improving a fuel saving property, a method of increasing a viscosity index of a lubricating oil by adding a viscosity index improver to a lubricating base oil has been known.
Moreover, for example, in the case of lubricating oils used for transmissions of vehicles, such as ATF, MTF, and CVTF (also referred to as “lubricating oils for a transmission” or “drive system oils”), as one means of improving a fuel saving property, there is a method of decreasing viscosity resistance by lowering the viscosity of a lubricating oil for a transmission. However, when the viscosity of a lubricating oil for a transmission is lowered, other problems such as oil leak and seizure may arise.
Therefore, as another method for improving a fuel saving property, there is a method involving use of a viscosity index improver. This method increases the viscosity index of a lubricating oil for a transmission by using a viscosity index improver, and suppresses the viscosity increase in a low-temperature region while maintaining the viscosity in a high-temperature region.
Regarding a viscosity index improver, the use of various viscosity index improvers has been proposed, and in particular, the use of poly(meth)acrylate-based viscosity index improvers has been often proposed (for example, refer to Patent Literatures 1 to 7).